1. Technical Field
The present invention is related to generation of water vapor. More specifically, the present invention is a method and apparatus for generating ultra-pure water vapor without utilizing an inert transporting gas.
2. Background Art
Ultra-pure water vapor is utilized in many applications and industries. One such use is for affixing a silicon oxide film by the water oxidation method in semiconductor and integrated circuit manufacturing processes. Another such use can be as an auxiliary reactant used during plasma photo resist stripping to aid in the removal of inorganic residues.
A first method to produce ultra-pure water vapor is a system to boil high purity water. Such a system is typically referred to as a boiler. FIG. 1 illustrates a prior art boiler system. The boiler system includes a chamber 100 for boiling the water, a water inlet 105, an upstream water purification and filtering system 110, a heat source 115, a water vapor outlet 120, an outlet pressure and flow controlling device 125, and a control and instrumentation system 130. Water is purified and filtered in the upstream water purification and filtering system 110 prior to introduction to the chamber 100. Heat is applied to the chamber 100 to produce steam (water vapor). The water vapor is then piped to the point of use through the outlet pressure and flow controlling device 125.
The boiler method of producing water vapor has several significant short falls that renders a boiler ineffective for use in semiconductor manufacturing operations.
First, a boiler system will concentrate the impurities contained in the water it is boiling. If the water flowing into the boiler has been purified and filtered to low parts per million (PPM) or better impurity levels, the majority of the trace impurities will remain in the chamber 100 when the water vapor is released. This leads to a concentration of impurities in the chamber 100. As time passes, the amount of impurities leaving the boiler can increase due to aerosols leaving with the saturated water vapor. These aerosols can be formed at the gas/liquid interface as vapor bubbles that rise to the surface, break and xe2x80x9csplatterxe2x80x9d liquid droplets into the vapor phase. Since these aerosols are formed from liquid in the boiler, the aerosols contain the same level of continuously increasing impurities. These high impurity levels can contaminate the product water vapor and the downstream delivery system.
Second, a boiler method is typically controlled by a feedback control process monitoring the pressure through the outlet pressure and flow controlling device 125 and adjusting the heat source 115, to maintain a constant pressure. This often results in oscillation and instability in the output flow, particularly when transient or non-steady state flow rates are required. This oscillation effect can further increase the formation of aerosols-described above.
A second method of producing high purity water vapor is referred to as a bubbler. FIG. 2 illustrates a prior art bubbler system. A bubbler consists of a sealed chamber 200 which is isolated from the out side air, a water inlet 205, an upstream water purification and filtering system 210, a heat source 215, a water vapor outlet 220, an inert gas inlet 230 and an inert gas flow control device 225.
The chamber 200 contains a quantity of water 235 therein, maintained at a freely selected constant temperature. An inert gas enters through the inert gas inlet 230 and passes through the water. The result is an inert gas which contains a water component corresponding to the vapor pressure of water at the freely selected temperature. The control of the water concentration is accomplished by means of the temperature and vapor pressure relationship within the chamber 200.
The bubbler method has several short falls. Accurate control of the water vapor concentration leaving the bubbler is dependant on the assumption that the carrier vapor achieves vapor liquid equilibrium with the bulk water. This requires accurate control of the liquid temperature, bubble sizes and distribution, bubble residence time in the water, and total operating pressure. In practice, simultaneous control of all these variables is difficult, and oscillations are likely to occur when transient or non-steady state flow rates are required. Obtaining pure water vapor is not possible with this method, due to the use of an inert gas to create the carrier bubbles. Impurity build-up similar to that experienced in a boiler system would also occur in this method.
A third method for generating high purity water vapor is one in which a standard gas contained in a cylinder is diluted. FIG. 3 illustrates a gas dilution type system. A dilution system includes a cylinder 300, containing a quantity of inert gas 305, with the inert gas having a known concentration of water vapor. A dilution system also includes an inert gas flow control 310, a diluent inert gas inlet 315, a diluent inert gas flow control 320, and an outlet 325.
In the dilution method, the inert gas 305 is diluted to a selected dilution ratio using a quantity of diluent inert gas from the diluent inert gas inlet 315. The water vapor concentration of the resulting gas mixture is determined by the inert gas flow control 310 and the diluent inert gas flow control 320.
The dilution method also has several shortfalls. First, the reliability of the water concentration is low since there are no standard gases having highly accurate water concentrations. Second, it i difficult to generate high concentrations and large quantities of water vapor, and by definition, generation of pure water vapor is not possible. The mixture being diluted cannot have a water concentration higher than the dew point at ambient conditions, or liquid condensation inside the storage container will result. Heating the container will increase the available concentration, but doing so is not practical in modem semiconductor fabs, and will also exhibit similar problems to that described in the boiler system above.
A fourth method of producing high purity water vapor is commonly referred to as combustion in a quartz diffusion furnace or, more simply, combustion. FIG. 4a illustrates a gas combustion system. A gas combustion system includes a combustion chamber 400, an ultra-pure oxygen inlet 405, an ultra-pure hydrogen inlet 410, an oxygen flow control 415, a hydrogen flow control 420, an outlet 425, a hydrogen gas nozzle 430, a Si chip 435 for ignition held in a vicinity of a top side of the hydrogen gas nozzle 430, and a heating lamp 440 for heating the Si chip 435.
A vicinity at the tip end of the hydrogen gas nozzle 430 inside the chamber 400 attains a high temperature from about 1800xc2x0 C. to 2000xc2x0 C. due to flames of combustion. In addition, the amount of oxygen gas supplied to the chamber 400 is set to a level exceeding one half that of the hydrogen gas in order to completely combust the hydrogen gas H2 and have excess oxygen remaining. This maintains safer operation of the system.
The combustion method achieves excellent practical effects in that high purity water is generated and can be instantaneously generated at a rate of several liters per minute. However, in this method, there is a problem in that if the flow rate of hydrogen gas or oxygen gas is reduced to decrease the water amount, combustion can easily be stopped. It is therefore, extremely difficult to provide controls for decreasing the amount of water vapor which is generated. The control range of a ratio of water vapor to oxygen is narrow. As a result, production of pure water vapor over wide pressure and flow rate ranges is very difficult, and may not be possible for systems requiring on/off flow rate demands.
The combustion method has an additional difficulty in that when combustion stops, raw gas is fed directly into the outlet 425. An interlock mechanism becomes indispensable to prevent a hydrogen gas explosion when combustion stops. This adds additional complexity and cost.
In addition, there is also a problem that when the gas flow rate is reduced, flames are generated in the vicinity of the nozzle 430, SiO2 material in the nozzle 430 begins to evaporate. SiO2 becomes volatile and mixes in the reactor atmosphere of H2O+O2 gas and contaminates the H2O+O2 gas fed to the semiconductor manufacturing equipment to such an extent that it is no longer suitable for use in manufacturing high-performance semiconductors. Yet another difficulty with the combustion method is the high temperature of the chamber 400. The chamber temperature can exceed 700xc2x0 C. and poses personnel safety risks.
FIG. 4b illustrates a modification to the gas combustion method. This modification limits the explosion concerns by mixing an inert gas with the hydrogen and oxygen gases. The modification adds an ultra-pure inert gas inlet 430, and an inert gas flow control 435. The inert gas can also be added to the hydrogen in a premixed, commercially available reagent known as xe2x80x9cforming gasxe2x80x9d. The ratio of an inert gas including argon to hydrogen is limited so that hydrogen does not exceed a relatively low level such as 8% and typically 4% to 6% of the resulting mixture. 8% hydrogen in air or oxygen containing atmospheres is generally accepted as the lowest concentration at which hydrogen could cause an explosion. This modification results in a safer combination of hydrogen and oxygen but also limits the water vapor produced to no more than the concentration of hydrogen. In addition, a large quantity of inert gas is introduced into the system and accompanies the water vapor out the outlet 425.
The FIG. 4b method reduces the risk of hydrogen explosion while everything is operating properly and all gases are flowing at the correct flow rates. This method does not address other combustion method short falls including: precise placement of gas injection nozzles; and high reactor temperatures. In addition, the output gas of this method is approximately 90% inert gas.
FIG. 4c illustrates a further modification to the combustion method. This modification adds a catalytic material 440 to the inner volume of the combustion chamber 400. This modification is described in detail in European Patent Application EP0878443A1 by Ohmi et al, (Ohmi) which is hereby incorporated by reference in it""s entirety. Ohmi teaches a method of mixing purified hydrogen gas and purified oxygen gas in a reactor similar to the combustion method described above, but limits the temperature of the reactor to below hydrogen""s auto-ignition temperature. To ensure complete reaction of the hydrogen, the reactor contains a catalyst material to aid the reaction. Ohmi also teaches the injection of an inert diluent gas to further enhance the safety of the process similar to that described in the FIG. 4b method described above. There are also commercially available systems produced by Fujikin of Japan that can produce nearly 100% water vapor from a catalytic reactor similar to that described by Ohmi. The Fujikin system, however, requires some excess oxygen for safety reasons, and therefore cannot produce high purity water vapor.
The FIG. 4c method reduces the risk of hydrogen explosion while everything is operating properly and all gases are flowing at the correct flow rates and temperatures. This method also reduces the reactor temperatures. This method adds complication and cost to the reactor by adding catalyst material coatings and gas diffusing components.
A fifth method of producing high purity water vapor is the diffusion tube method. FIG. 5 illustrates a prior art diffusion tube system. A diffusion tube system includes a diffusion tube 500, containing a quantity of porous quartz, ceramics or plastic resin material 505, a water inlet 510, an inert gas inlet 515, an inert gas and water vapor outlet 520, and a heat source 525. In this method, water is introduced into the resin material 505 in a diffusion tube 500 which is permeated by water molecules.
In the diffusion tube method, liquid water molecules migrate through the resin material 505 to an end surface 530 of the diffusion tube 500 which is exposed to the inert gas flow. A thin, liquid water film forms on the evaporating end surface 530 and then evaporates into the inert gas flowing past the end surface 530 of the diffusion tube 500. Thus water vapor is generated. The control of the water concentration is determined by the temperature of the diffusion tube, the porosity of the resin material 505 and the flow rate of the inert gas.
The diffusion tube method has several short falls. First, it is difficult to consistently control the evaporation rate. Similar to the bubbler method described in FIG. 2 above, the accuracy of the outlet water vapor concentrations relies on the assumption of obtaining vapor/liquid equilibrium at a known temperature. Control of the interface temperature is critical, but difficult, especially with non conductive porous media such as resins or quartz. The use of metal porous tubes is not recommended for purity reasons. Second, at low evaporation rates, liquid water can seep through and form droplets on the evaporating surface 530 and in the inert gas and water vapor outlet 520. Controlling water droplet formation would require additional controls, complexity and cost.
Third, as with other methods described above, delivery of transient flow rates over a wide range of pressures is also difficult. Fourth, to achieve higher evaporation rates requires large quantities of energy to the evaporating end surface 530. With plastic or quartz diffusion tubes that are basically insulating, this is very difficult to control. Heating the gas is possible, but not practical, due to the typically poor (low) heat capacity of most gasses compared to the high heat of vaporization of water, and the large variation of temperature that would result along the length of the tube. Fifth, diffusion tube 500 properties can change over time, so that the quantity of water vapor produced at a given temperature can vary. This variation over time is due, in part, to the deposit of non volatile impurities present in the liquid water in the pores leading to and immediately at the evaporation surface. Sixth, it is not possible to produce pure water vapor with this method as an inert carrier gas is required.
What is needed is a method and apparatus which is easy to control, delivers repeatable results, is capable of generating substantially 100% concentrations of ultra-pure water vapor, at high flow rates, at atmospheric pressures and above atmospheric pressures, at reduced reactor temperatures, reducing or eliminating the hydrogen explosion or auto-ignition hazards of combustion and without requiring an inert diluent gas in the delivered water vapor.
The present invention increases the safety of a reactor for generating water vapor from oxygen and hydrogen. The present invention provides ultra-pure water vapor in an amount necessary for practical use safely, stably and continuously, provides ultra-pure water vapor concentrations to nearly 100 percent without the need of an inert carrier gas. The purity of water vapor is not dependent on flow rate demands or transient processes.
A method of producing ultra-pure water vapor includes combining a quantity of ultra-pure hydrogen, an excess quantity of ultra-pure oxygen and a quantity of an ultra-pure inert gas in the presence of a catalyst. The resulting mixture of water vapor, excess oxygen and inert gas flows from the catalyst to a water vapor sorption material. The water vapor sorption material sorbs water vapor from the mixture and the remaining inert gas and excess oxygen flows out through a waste vent outlet. When the water vapor sorption material has absorbed a quantity of water vapor, the flow of the mixture into the water vapor sorption material is stopped. Then the water vapor sorption material is heated to cause the water vapor sorption material to release the water vapor. High concentrations and large quantities of pure water vapor then flow out of the outlet.
An apparatus for producing ultra-pure water vapor includes a reactor vessel and a plurality of sorption vessels. The reactor vessel is constructed from a heat-resistant material and includes an inlet and an outlet for water vapor and inert gas-mixture, a heat source, and has an oxidation catalyst within the reactor vessel. The sorption vessels are made of a heat-resistant material and include an inlet and an outlet for water vapor and inert gas mixture, a heat source, and have a water vapor sorption material within the sorption vessel.
Hydrogen, oxygen and an inert gas fed from the inlet of the reactor vessel contacts the catalyst to enhance reactivity, thereby producing water from hydrogen and oxygen. The water vapor and inert gas mixture flows from the reactor vessel to the sorption vessel, where the water vapor, excess oxygen, and inert gas mixture contacts the sorption material. The sorption material sorbs the water vapor from the excess oxygen and inert gas mixture. The inert gas and excess oxygen are released to an exhaust vent. Then, the water vapor and inert gas mixture flowing from the reactor vessel to the sorption vessel is stopped. The sorption vessel is heated to release ultra-pure water vapor. The temperature of the sorption material determines the pressure of the water vapor in the adsorption vessels. The flow rate of water leaving the system can be controlled by mass flow controllers or valves. Since the quantity of water remaining in the sorption vessels will decrease over time and usage, a heating control system will continuously is increase the sorption vessel temperature to maintain a constant vapor pressure consistent with the delivery demands designed into the system.
The present invention provides improved, ultra-pure water vapor generation over previous technologies. The innovative design advances the state of the art of ultra-pure water vapor generation with the advantages of increased safety, substantially 100% ultra pure water vapor at the outlet, high flow rates, at atmospheric pressures and greater than atmospheric pressures, with reduced reactor temperatures, ease of control and repeatability of results over the prior art technologies without the disadvantages of hydrogen auto-ignition or explosion hazards or an inert diluent gas in the delivered water vapor.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading the following detailed descriptions and studying the various figures and drawings.